This invention relates to a cylinder device and, more particularly, to improvements in a seal for the piston of the cylinder device for effecting good sealing characteristics during normal usage and also during a vacuum oil-filling operation.
In a conventional hydraulic cylinder device such as a master cylinder or the like, a piston is slidably fitted in a cylindrical bore formed in a main body of the cylinder device to partition the interior of the bore into an oil space and an air space, and a seal in the form of a molded packing ring such as a U-packing or a U-cup packing is fitted on the piston with a lip portion thereof being located adjacent to and angled toward the oil space and engaging with the inner periphery of the bore to seal the oil chamber from the air space.
In a modified form, an additional lip is formed on or adjacent the end toward the air space to improve the sealing characteristics of the seal. The additional lip is spaced from the first mentioned lip in the direction of the axis of the bore and is also angled toward the oil space and normally the first mentioned lip in the direction of the axis of the bore and is also angled toward the oil space and normally engaged with the bore thus augmenting the sealing characteristics of the first mentioned lip and, further, additional lip is effective to prevent the seal from being subjected to the so-called rolling phenomenon.
Such a seal acts satisfactorily in the usual operating conditions.
However, there has been proposed a process for filling hydraulic liquid into a newly assembled hydraulic system of a vehicle, wherein the interior of the hydraulic system including various components and pipe lines is firstly evacuated to a high vacuum and, thereafter, hydraulic liquid is supplied into the system thus eliminating troublesome air-bleeding operations, and such a filling operation is usually referred to as a vacuum oil-filling operation.
The conventional lip type seal is not effective in a vacuum oil-filling operation, since the high vacuum prevailing in the oil space tends to rollingly move the seal toward the oil chamber to separate the lip from the wall of the bore, because the lip is angled toward the oil space to act against high pressure in the oil chamber and, moreover, the pressure difference acting across the seal in the high vacuum condition is in the opposite direction to the normal pressure difference.